rename WindowedValue.getPane() to WindowedValue.getPaneInfo() in preparation for making it public

[kennknowles]

This is a preliminary change to reduce the number of files to review in #34902

For the user-facing API I think we would like getPaneInfo to be appropriately named. Notably, a PaneInfo record is not the same as a "pane". A "pane" is a name for a single triggered output of a window (probably we tried to be too clever with names) but the PaneInfo does not uniquely identify the pane. In Python SDK this is the public field WindowedVAlue.pane_info so it also makes the SDKs match better. In Go SDK it is pane but it is not blocking the current change.

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[kennknowles]

R: @stankiewicz

since you noticed this was a lot of the files in that PR

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[kennknowles]

Also @talatuyarer commented the same :-)

[stankiewicz]

thank you for splitting the PR :)

[kennknowles]

It looks like there might be a need to do this in multiple steps or with a complex dance, due to containers and/or reflection. I think the adapter method that I put in #35140 will work, if I merge that one first.

[kennknowles]

Thanks for reviewing, anyhow! I will do this in a small step after #35140. If you can review that it would be great. It is "large" but has almost no actual content.